1. Field of the Invention
The invention concerns X-ray tube cooling devices.
2. Description of the Prior Art
X-ray tubes, for medical diagnosis for example, are generally formed by a diode, namely, with a cathode and an anode or an anti-cathode, these two electrodes being enclosed in a vacuum sealed casing enabling the setting up of electrical insulation between these two electrodes. The cathode produces a beam of electrons and the anode receives these electrons on a small area which forms a focal spot from which the X-rays are emitted.
When the high supply voltage is applied to the terminals of the cathode and the anode so that the cathode is at the negative potential, a so-called anode current is set up in the circuit through a generator producing the high supply voltage. The anodic current crosses the space between the cathode and the anode in the form of an electron beam which bombards the focal spot.
A small portion of the energy used to produce the electron beam is converted into X-rays, while the rest of this energy is converted into heat. Thus, also taking into account the high instantaneous power values involved (of the order of 100 KW) and the small dimensions of the focal spot (of the order of 1 millimeter), manufacturers have long been making rotating-anode X-ray tubes where the anode is made to rotate to distribute the thermal flux on a crown or ring, called a focal ring, which has a far greater area than the focal spot, the interest of this approach being all the greater as the rotational speed is high (generally between 3,000 and 12,000 rpm).
The standard type of rotating anode has the general shape of a disk with an axis of symmetry around which it is made to rotate by means of an electrical motor. The electrical motor has a stator located outside the casing and a rotor which is mounted in the casing of the X-ray tube and arranged along the axis of symmetry, the rotor being mechanically joined to the anode by means of a supporting shaft.
The energy dissipated in a tube of this type is high and, therefore, provision is made to cool it. To this end, the tube is enclosed in a chamber in which a cooling fluid, notably oil, is made to flow. The fluid is itself cooled in an exchanger which may be of the air or water type. Thus, a cooling device has been made that works permanently. However, the X-ray tube emits only intermittently so that the dissipated energy is high during the examination phase itself, which lasts from a few seconds to a few minutes, and is practically null during the resting time needed for changing patients. The result is major disparities in the quantity of heat to be removed depending on the phase considered. This leads notably to major variations in the temperatures of the tube materials used. These variations may harm the operation of the tube. Thus, the variations in the tube fixing part cause shifts in the focal spot. The cooling chamber or sheath may also undergo major variations in temperature. This is harmful to the environment especially when there are electronic devices nearby. To prevent a great increase in temperature during the examination period, it has been proposed to increase the refrigeration capacity of the cooling device, but this leads to the oversizing of the latter, which is incompatible with the available space.
Furthermore, during the resting stage, a major drop in temperature is obtained. This drop is all the greater as this stage is long, with the above-mentioned drawbacks relating to major variations in temperature.
To overcome this latter aspect, it has been proposed to regulate the operating time of the cooling device as a function of the temperature of the cooling fluid. A regulation of this kind, which appears to be easy to implement in theory, is difficult to achieve in practice for technical and technological reasons which shall not be explained herein.